Design-based monitoring

ABSTRACT

A method for monitoring fabrication of an integrated circuit (IC) on a semiconductor wafer includes generating a product design profile (PDP) using an electronic design automation (EDA) tool, the PDP comprising an indication of a site in a layer of the IC that is susceptible to a process fault. Upon fabricating the layer of the IC on the wafer, a process monitoring tool is applied to perform a measurement at the site in the layer responsively to the PDP.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.10/780,377, filed Feb. 17, 2004 which claims the benefit of U.S.Provisional Patent Application 60/486,565, filed Jul. 11, 2003, which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to process monitoring, andspecifically to methods and apparatus for inspection and testing ofsemiconductor wafers and reticles in production.

BACKGROUND OF THE INVENTION

Electronic design automation (EDA) tools are applied extensively in thedesign of integrated circuits and in verification of these designs. EDAtools are used, for example, in logic synthesis, placement and routingof circuit elements, circuit layout and design rule checking. At the endof the design process, a set of reticles (also referred to as masks ortemplates) is generated for use in producing the integrated circuit(IC). Each reticle corresponds to one layer of the thin film structuresthat will be fabricated on a semiconductor wafer to produce the IC.

In the fabrication facility (fab), the reticles are used to print thesuccessive circuit layers on the wafer in a photolithographic process.Each wafer undergoes hundreds of processing steps, including not onlyprinting the reticles, but also material deposition, etching, cleaningand polishing. Defects in any of the process steps can substantiallyreduce the process yield. Therefore, monitoring tools are used to checkwafers at nearly every step in the process to verify that the processingtools are functioning properly. Monitoring tools that are used for thispurpose include systems for inspection and metrology of integratedcircuit structures (typically optical or electron beam-based), as wellas systems for electrical testing of circuit components. Reticles arealso monitored for defects that may result in defects on the wafer. Inthe context of the present patent application and in the claims, theterm “monitoring” should be understood as including all availablemodalities for testing wafers and reticles in production for purposes ofdetecting process or product defects.

U.S. Pat. No. 6,529,621, whose disclosure is incorporated herein byreference, describes mechanisms for making and inspecting reticles,based on a reusable circuit design for use with EDA tools. The patentpoints out that conventional inspection systems waste valuable resourcesby inspecting some regions of the reticle too stringently, and notreliably inspecting other regions stringently enough. Conventionalinspection systems and techniques are unable to distinguish betweencritical and non-critical areas of the reticle. The patent thereforedescribes mechanisms for flagging critical or noncritical regions of anIC pattern database. Other design flow procedures, such as reticleproduction and inspection and IC device fabrication, may then be basedon the flagged critical or noncritical areas in the database.

SUMMARY OF THE INVENTION

Process engineers in the semiconductor fab are faced with conflictingneeds: Although process monitoring should be as thorough as possible toprovide early detection of all process defects, testing in practice mustoften be limited to a small number of locations on the wafer so as notto compromise the process throughput. In general, the process engineerselects the locations to test based on ad hoc recipes. Given the extremecomplexity of modern IC designs and production processes, it is almostinevitable that this heuristic approach will fail to select the optimalset of locations for testing.

In response to this difficulty, the present invention provides methodsand systems for applying design information generated by EDA tools inprocess monitoring. In embodiments of the present invention, EDA toolsare used to assemble a product diagnostic profile (PDP) during thedesign of an IC. For each IC die, and each reticle in the set used toproduce the die, the PDP contains various categories of data for use inguiding the associated process monitoring steps. Typically, the datainclude targets (used for wafer alignment in inspection tools), specialtest structures, classification of regions of the wafer based ongeometrical and/or functional qualities, and identification of criticalsites for measurement during production. Additionally or alternatively,other data types may be included in the PDP.

The PDP data are transferred to the fab for use by process engineers indefining the monitoring steps to be performed on each die at each stagein production. Typically (although not necessarily), the PDP data aretransferred together with the design of the reticle set to which thedata apply. When different dies are combined in a common reticle set (sothat multiple different ICs are produced on the same wafer), the PDPdata may likewise be combined for monitoring purposes. The PDP dataassist the process engineer in determining the appropriate test typesand recipes to be used at each production stage and in choosing thelocations to test. Judicious use of the PDP data can maximize thesensitivity of detection of process deviations and enable earlydiscovery of process faults, while minimizing the impact of testing onprocess throughput.

Although the embodiments described hereinbelow relate specifically totests that are applied to IC patterns on semiconductor wafers, theprinciples of the present invention may similarly be applied to testingof the reticles that are used in production of the wafers.

There is therefore provided, in accordance with an embodiment of thepresent invention, a method for monitoring fabrication of an integratedcircuit (IC) on a semiconductor wafer, including:

generating a product design profile (PDP) using an electronic designautomation (EDA) tool, the PDP including an indication of a site in alayer of the IC that is susceptible to a process fault;

fabricating the layer of the IC on the wafer; and

applying a process monitoring tool to perform a measurement at the sitein the layer responsively to the PDP.

In one embodiment, applying the process monitoring tool includesmeasuring a dimension associated with one or more features of the IC atthe site.

In another embodiment, generating the PDP includes making adetermination that the site is critical to a performance rating of theIC, and selecting the site responsively to the determination.

In yet another embodiment, generating the PDP includes making adetermination that the site is marginal with respect to a design rule ofthe IC, and selecting the site responsively to the determination.Alternatively or additionally, generating the PDP includes making adetermination that the site is marginal with respect to a variation in aparameter of a process used in fabricating the IC, and selecting thesite responsively to the determination.

In still another embodiment, generating the PDP includes determining amask error enhancement factor (MEEF) at the site, and selecting the siteresponsively to the MEEF. Alternatively or additionally, generating thePDP includes determining an optical proximity correction (OPC) to beapplied at the site, and selecting the site responsively to the OPC.

In a further embodiment, generating the PDP includes determining adensity of structures in the IC at the site, and selecting the siteresponsively to the density.

In one embodiment, the site includes a location of a pair of matchedcircuit elements, and applying the process monitoring tool includesverifying that a critical characteristic of both the circuit elements inthe pair is substantially identical.

In another embodiment, the method includes predicting a yield of thefabrication of the IC responsively to the PDP and to the measurement.

There is also provided, in accordance with an embodiment of the presentinvention, a method for monitoring fabrication of an integrated circuit(IC) on a semiconductor wafer, including:

generating a product design profile (PDP) using an electronic designautomation (EDA) tool, the PDP including an identification of a regionin a layer of the IC that is characterized by a periodic pattern;

fabricating the layer of the IC on the wafer; and

applying a process monitoring tool to perform a measurement in theregion of the layer responsively to the periodic pattern.

In some embodiments, generating the PDP includes determining a pitch anda direction of the periodic pattern, and applying the process monitoringtool includes selecting a spatial filter responsively to the pitch andthe direction, and performing optical inspection of the region using thespatial filter.

In another embodiment, generating the PDP includes determining adirection of the periodic pattern, and applying the process monitoringtool includes selecting a scan direction responsively to the directionof the periodic pattern, and inspecting the region while scanning overthe region in the selected scan direction.

In yet another embodiment, generating the PDP includes determining anexact period of a repetitive feature in the periodic pattern, andapplying the process monitoring tool includes capturing multiple imagesof the feature at locations on the wafer that are mutually spaced by theexact period, and comparing each of the images to another of the imagesor to a reference image.

In a further embodiment, applying the process monitoring tool includesdetermining, responsively to the periodic pattern, a sensitivity settingto be applied by the process monitoring tool in detecting defects in theregion, wherein different sensitivity settings are applied by theprocess monitoring tool in different regions of the layer.

There is additionally provided, in accordance with an embodiment of thepresent invention, a method for monitoring fabrication of an integratedcircuit (IC) on a semiconductor wafer, including:

generating a product design profile (PDP) using an electronic designautomation (EDA) tool, the PDP including an identification of aplurality of regions in a layer of the IC and a respective criticalityparameter for each of the regions, indicative of a maximum tolerabledefect size in each of the regions;

fabricating the layer of the IC on the wafer; and

applying a process monitoring tool to perform a measurement in one ormore of the regions in the layer responsively to the respectivecriticality parameter.

Typically, applying the process monitoring tool includes setting adefect detection threshold in each of the one or more of the regionsresponsively to the respective criticality parameter. Additionally oralternatively, applying the process monitoring tool includes selectingthe one or more of the regions to inspect responsively to the respectivecriticality parameter. Further additionally or alternatively, applyingthe process monitoring tool includes detecting a defect in one of theregions, and classifying the defect responsively to the criticalityparameter.

There is further provided, in accordance with an embodiment of thepresent invention, a method for monitoring fabrication of an integratedcircuit (IC) on a semiconductor wafer, including:

designing a layout of a layer of the IC using an electronic designautomation (EDA) tool, the layer including a structure that is amenableto testing;

generating a product design profile (PDP) using the EDA tool, the PDPincluding information regarding the structure;

fabricating the layer of the IC on the wafer; and

applying a process monitoring tool to perform a measurement on thestructure in the layer, responsively the information in the PDP.

Typically, the structure includes a dedicated test structure, whereinthe dedicated test structure is located in a non-die area of the wafer.

In some embodiments, the structure includes multiple elongate parallelconductors, and applying the process monitoring tool includes testing anelectrical continuity of the conductors. Typically, testing theelectrical continuity includes applying an electrical charge at a firstend of at least some of the conductors, and measuring the electricalcharge at a second end of the conductors.

In another embodiment, the structure includes one or more contactopenings in the layer, and applying the process monitoring tool includesdirecting an electron beam to irradiate the one or more contactopenings, and measuring a specimen current responsively to the electronbeam.

There is moreover provided, in accordance with an embodiment of thepresent invention, apparatus for producing an integrated circuit (IC) ona semiconductor wafer, including:

an electronic design automation (EDA) tool, which is adapted to generatea design of a layer of the IC and a product design profile (PDP), whichincludes an indication of a site in the layer that is susceptible to aprocess fault;

a production tool, which is adapted to fabricate the layer of the IC onthe wafer responsively to the design; and

a process monitoring tool, which is adapted to perform a measurement atthe site in the layer responsively to the PDP.

In one embodiment, the apparatus includes a computer processor, which isadapted to predict a yield of the fabrication of the IC responsively tothe PDP and to the measurement.

There is furthermore provided, in accordance with an embodiment of thepresent invention, apparatus for producing an integrated circuit (IC) ona semiconductor wafer, including:

an electronic design automation (EDA) tool, which is adapted to generatea design of a layer of the IC and a product design profile (PDP), whichincludes an identification of a region in a layer of the IC that ischaracterized by a periodic pattern;

a production tool, which is adapted to fabricate the layer of the IC onthe wafer responsively to the design; and

a process monitoring tool, which is adapted to perform a measurement inthe region in the layer responsively to the periodic pattern.

There is also provided, in accordance with an embodiment of the presentinvention, apparatus for producing an integrated circuit (IC) on asemiconductor wafer, including:

an electronic design automation (EDA) tool, which is adapted to generatea design of a layer of the IC and a product design profile (PDP), whichincludes an identification of a plurality of regions in a layer of theIC and a respective criticality parameter for each of the regions,indicative of a maximum tolerable defect size in each of the regions;

a production tool, which is adapted to fabricate the layer of the IC onthe wafer responsively to the design; and

a process monitoring tool, which is adapted to perform a measurement inone or more of the regions responsively to the respective criticalityparameter.

There is additionally provided, in accordance with an embodiment of thepresent invention, apparatus for producing an integrated circuit (IC) ona semiconductor wafer, including:

an electronic design automation (EDA) tool, which is adapted to generatea design of a layer of the IC, the layout including a structure that isamenable to testing, and which is further adapted to generate and aproduct design profile (PDP), which includes information regarding thestructure;

a production tool, which is adapted to fabricate the layer of the IC onthe wafer responsively to the design; and

a process monitoring tool, which is adapted to perform a measurement onthe structure, responsively the information in the PDP.

There is further provided, in accordance with an embodiment of thepresent invention, a computer software product for use in producing anintegrated circuit (IC) on a semiconductor wafer, the product includinga computer-readable medium in which program instructions are stored, theinstructions including at least one of an electronic design automation(EDA) program component and a process monitoring program component,

wherein the EDA program component, when read by a computerized EDA tool,causes the EDA tool to generate a design of a layer of the IC and aproduct design profile (PDP), which includes an indication of a site inthe layer that is susceptible to a process fault, and

wherein the process monitoring program component, when read by acomputerized process monitoring tool after fabrication of the layer ofthe IC on the wafer responsively to the design, causes the processmonitoring tool to perform a measurement at the site in the layerresponsively to the PDP.

There is moreover provided, in accordance with an embodiment of thepresent invention, a computer software product for use in producing anintegrated circuit (IC) on a semiconductor wafer, the product includinga computer-readable medium in which program instructions are stored, theinstructions including at least one of an electronic design automation(EDA) program component and a process monitoring program component,

wherein the EDA program component, when read by a computerized EDA tool,causes the EDA tool to generate a design of a layer of the IC and aproduct design profile (PDP), which includes an identification of aregion in a layer of the IC that is characterized by a periodic pattern,and

wherein the process monitoring program component, when read by acomputerized process monitoring tool after fabrication of the layer ofthe IC on the wafer responsively to the design, causes the processmonitoring tool to perform a measurement in the region in the layerresponsively to the periodic pattern.

There is furthermore provided, in accordance with an embodiment of thepresent invention, a computer software product for use in producing anintegrated circuit (IC) on a semiconductor wafer, the product includinga computer-readable medium in which program instructions are stored, theinstructions including at least one of an electronic design automation(EDA) program component and a process monitoring program component,

wherein the EDA program component, when read by a computerized EDA tool,causes the EDA tool to generate a design of a layer of the IC and aproduct design profile (PDP), which includes an identification of aplurality of regions in a layer of the IC and a respective criticalityparameter for each of the regions, indicative of a maximum tolerabledefect size in each of the regions, and

wherein the process monitoring program component, when read by acomputerized process monitoring tool after fabrication of the layer ofthe IC on the wafer responsively to the design, causes the processmonitoring tool to perform a measurement in one or more of the regionsresponsively to the respective criticality parameter.

There is also provided, in accordance with an embodiment of the presentinvention, a computer software product for use in producing anintegrated circuit (IC) on a semiconductor wafer, the product includinga computer-readable medium in which program instructions are stored, theinstructions including at least one of an electronic design automation(EDA) program component and a process monitoring program component,

wherein the EDA program component, when read by a computerized EDA tool,causes the EDA tool to generate a design of a layer of the IC, thelayout including a structure that is amenable to testing, and a productdesign profile (PDP), which includes information regarding thestructure, and

wherein the process monitoring program component, when read by acomputerized process monitoring tool after fabrication of the layer ofthe IC on the wafer responsively to the design, causes the processmonitoring tool to perform a measurement on the structure, responsivelythe information in the PDP.

There is additionally provided, in accordance with an embodiment of thepresent invention, apparatus for monitoring production of an integratedcircuit (IC) on a semiconductor wafer, following fabrication of a layerof the IC on the wafer responsively to a design and a product designprofile (PDP) generated by an electronic design automation (EDA) tool,the PDP including an indication of a site in the layer that issusceptible to a process fault, the apparatus including:

a process monitoring tool, which is adapted to perform a measurement atthe site in the layer responsively to the PDP.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, pictorial illustration of a system for IC designand fabrication, in accordance with an embodiment of the presentinvention;

FIGS. 2 and 3 are block diagrams that schematically illustrate datastructures in a product diagnostic profile, in accordance with anembodiment of the present invention;

FIG. 4 is a schematic top view of a die in a semiconductor wafer showingdivision of the die into regions for monitoring purposes, in accordancewith an embodiment of the present invention;

FIG. 5 is a schematic detail view showing structures formed on asemiconductor wafer and a test site associated with the structures, inaccordance with an embodiment of the present invention; and

FIG. 6 is a schematic top view of a structure on a semiconductor waferthat is used in electrical testing of the wafer, in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS System Overview

FIG. 1 is a schematic, pictorial illustration of a system 20 for ICdesign and fabrication, in accordance with an embodiment of the presentinvention. A design engineer, using EDA tools on a design workstation22, designs elements of the IC, from the logic synthesis down to thespecific layout of circuit components. The design is stored inelectronic form in a design database 24. The database specifies the setof reticles that are to be used in producing the IC. In addition,workstation 22 generates, as a by-product of the IC design, a productdiagnostic profile (PDP), which is also held in the design database. ThePDP, which is described in detail hereinbelow, is typically generatedinteractively by the design engineer on workstation 22. Alternatively oradditionally, elements of the PDP may be generated by the workstationautomatically. Further alternatively or additionally, a separatecomputer (not shown) may be used to generate all or part of the PDPbased on the information in database 24.

A verification engineer checks the design in database 24 using averification workstation 23. Such verification typically includes, forexample, design rule checking, as well as other means known in the artfor physical and functional design verification. Workstation 23 may alsoadd data to the PDP, automatically or through interaction with theverification engineer.

Table I below shows a typical EDA work flow, from initial circuitspecification through to tape-out. The flow steps are shown here by wayof example, as background for creation of the PDP, which is describedbelow. Those skilled in the art will recognize that there may be manyvariations and additions to the sequence of steps listed below, and thataccordingly there may be variations, as well, in the methods used toderive the PDP data from different EDA tools.

TABLE I EDA FLOW STEPS Step Function Behavioral modeling Checkfunctional aspects of design Simulation Check for design accuracy andfaults Logic synthesis Create net list Timing analysis Check criticalpath and speed requirements Place and route Automated circuit layoutLayout Manual changes to the circuit layout RC extraction Checkparasitic capacitance and resistance in the circuit Layout vs. schematicCompare layout to net list design Design rule check Verify compliancewith design rules Stream GDS2 Store design data in a geometricalpolygon-based architecture OPC Optical proximity corrections - toaccommodate optical artifacts in photolithography Tape-out Complete maskdesign

When the design is complete, it is transferred to the fab forproduction. A reticle writer 26 generates a set of reticles 28 based onthe design in database 24. The reticles are then used in a fab processtool 30 to produce IC dies 34 on a silicon wafer 32. Of course, thisview is grossly oversimplified, and multiple different process steps aretypically applied to each layer on the wafer. In other words, eachreticle 28 maps to a sequence of several process steps in the fab.

After each process step, wafer 32 may be inspected or otherwise testedby a monitoring tool 36. Typically, various different monitoring toolsare used at different steps in the process. For example, a scanningelectron microscope (SEM), such as the SEMVision™ or NanoSEM™ system,produced by Applied Materials (Santa Clara, Calif.), may be used fordefect review and/or critical dimension measurements. As anotherexample, an optical inspection system, such as the Compass™ or ComPLUS™system, also produced by Applied Materials, may be used for bright-fieldor dark-field defect inspection. Alternatively, other types ofinspection tools may be used, as may electronic test instruments, as areknown in the art. Some particular test applications are described belowin greater detail.

The specific types of tests to be performed by monitoring tool 36, andthe locations on dies 34 at which the tests are to be applied, areselected based on data from the PDP in design database 24. Typically, aprocess engineer uses a monitoring workstation 38 to select and set upthe tests interactively, based on the PDP data. The test results may bereviewed on workstation 38. When a defect or other process deviation isobserved by monitoring tool 36, the results are used in makingadjustments to process tool 30 in order to correct and avoid defects insubsequent wafers. The results may also be used for proactivemonitoring—to refine the test procedure so as to focus on sites andregions that are known to be susceptible to defects.

Workstations 22, 23 and 38 typically comprise general-purpose computers,running suitable software for the purposes described hereinbelow.Monitoring tool 36 typically comprises a computer processor, as well,with suitable software for carrying out testing and inspection functionsbased on the PDP data in the design database. Thus, software for thepurposes of the present invention typically comprises an EDA programcomponent, for use on workstations 22 and/or 23, and a processmonitoring program component, for use on workstation 38 and/ormonitoring tool 36. The software for the workstations and the monitoringtool may be provided in electronic form, over a network, for example, orit may alternatively be supplied on tangible media, such as CD-ROM.

Data Structures

FIG. 2 is a block diagram that schematically illustrates a datastructure 40 that is used to hold PDP data, in accordance with anembodiment of the present invention. As noted above, the data instructure 40 are typically produced by a combination of automated andinteractive steps in various stages of IC design and verification.Preferably, the data in structure 40 are written in a unified,predefined format, to allow the data to be exchanged between differentdesign, verification and monitoring tools without additional translationsteps. For example, the data format may be based on XML tags.

Structure 40 relates to a “product” 42, which corresponds to a set ofreticles 28 that are used in the fab. The reticles may themselvesinclude multiple dies, which may be multiple instances of the same IC ormay belong to different ICs. Data structure 40, however, will generallycontain multiple die records 44 only if a single reticle set is used toproduce multiple different ICs. The data structure for each die 44encapsulates the diagnostic information relating to the particular IC towhich the die belongs. In addition, reticles typically comprise non-dieareas 46 (scribe lines) in between dies 44. These non-die areas maycontain features used for testing and alignment of the wafer.Information regarding these features is held in data structure 40 alongwith the die information.

For each die 44 (and each non-die area 46), structure 40 contains a setof masks 48, each corresponding to one of the reticles that are used inproducing the die. Each mask 48 thus corresponds to a singlephotolithographic patterning step. As noted above, there may be multipleprocess steps associated with each mask. For each mask 48, the followingcategories of diagnostic data are provided:

Alignment targets 50—These are structures that are used for precisealignment of wafers in monitoring tools (as well as other process tools,such as steppers and scanners).

Special structures 52—These structures are typically (although notnecessarily) placed in non-die areas, for use in specific testingfunctions related to the manufacturing process. Examples of specialstructures include structures that can be accessed for external probing,using electrical probes or suitable probes of other types. Other specialstructures include focus-exposure matrices and test structures forcharacterization of chemical-mechanical polishing (CMP) steps andoptical proximity corrections (OPC).

Regions 54—These are relatively large areas, typically within die 44,that share some geometrical or functional quality that can be used bymonitoring tools to optimize productivity, sensitivity, throughput orquality of the information that is generated in a given testing step.Details of region information are shown below in FIG. 3.

Sites 56—These are specific locations, typically expressed in X-Ycoordinates, that have particular diagnostic significance. These sitesand the measurements or other requirements pertaining to the sites maybe specified explicitly by the IC design and verification engineersduring the design of the IC reticle set. Alternatively or additionally,such sites may be identified automatically by EDA tools. Typically,sites 56 are chosen based on requirements relating to circuit functionsand/or manufacturability. The listing of sites enables designers tospecify critical locations for measurement and acceptable measurementtolerances at the locations.

The above categories are listed by way of example, and other categoriesmay be added to structure 40.

FIG. 3 is a block diagram that schematically shows details of the datastructure used to hold information regarding regions 54 in the PDP, inaccordance with an embodiment of the present invention. The regions of agiven mask 48 may be segmented according to various differentsegmentation schemes, each based on a different classificationparameter. Each segmentation scheme divides the reticle area intosections 72, depending on the classification parameter that is chosen. Agiven location on a particular die may thus belong to multiple differentsections in different segmentation schemes.

The following exemplary segmentation schemes are shown in the figure:

Exact period 60—In an exact period section of the die, all structuresare repetitive with the same spatial period. If high-resolution images(such as SEM images) of the section are taken at locations that arespaced by multiples of the period, the images should match exactly whenoverlaid one on another. In other words, a single patch of the regioncan be used as a reference for comparison of any other patch. Memorycells are a notable example of exact period structures. Typically, exactperiod segmentation is most useful in sections whose period rangesbetween 2 and 40 μm.

Pitch 62—Pitch sections are defined by a characteristic period in theirspatial structure, although the structure is not necessarily repetitive.Pitch can be particularly important, for example, in determining optimaloptical inspection parameters, as described hereinbelow.

Function 64—These sections are defined by their functionality, asdefined by the die “floor plan.” By way of example, functional sectionsmay typically include:

-   -   SRAM array    -   DRAM array    -   Flash memory array    -   Column decoder    -   Row decoder    -   Redundancy circuit    -   Logic    -   JTAG (Joint Test Action Group)    -   Line drivers    -   Parallel bus    -   Analog amplifier    -   RF amplifier    -   User-defined sections

Criticality 66—Segmentation by criticality depends on the sensitivity ofdifferent sections of the die to defects. The criticality classificationof a section may depend, for example, on the maximum tolerable defectsize in the section.

Redundancy 68—Certain regions of the die, such as memory arrays, havebuilt-in redundancy, so that a certain number of defects can betolerated without negative impact on the circuit. Other regions, such asdecoder logic, may not include redundant elements. Redundancy sectionsmay be classified by a redundancy parameter N, indicating the number ofdefective features that may be present in the section without causingdie failure.

User-defined segmentation 70—Additional segmentation schemes may bedefined by design, verification or process engineers.

The above segmentation schemes are listed here by way of example, andfurther schemes may be added to regions 54.

FIG. 4 is a schematic top view of one of dies 34, showing segmentationof the die into regions, in accordance with an embodiment of the presentinvention. Typically, die 34 comprises multiple cells 82, some occurringin repetitive patterns. The cells are hatched in the figure to showdifferent sections of the die, based on various segmentation criteria.For example, a region 84 of the die may comprise memory, and thus may beclassified by function. Another region 86 (which may overlap with region84) may have a certain exact period, and may thus be classified as anexact period section. Yet another region 88 may comprise logic or analogcircuits. The locations that are chosen for testing and the applicabletest parameters at each stage in the production of die 34 are chosenbased on the region segmentation of the mask that is used at that stage.Certain regions may have a higher concentration of test locations, whileother regions may have fewer or no locations examined in a given test.

In the data structure for sites 56 (FIG. 2), each site record holds alist of one or more coordinate locations within the corresponding mask48, along with a set of monitoring attributes that apply to all of theselocations. The site record may also include a reference image of thesite, for comparison against images of the site on the actual die. Otherfields that may be contained in the site record—as provided by thedesign or verification engineer for use by the process engineer—includethe following:

Device type (of the device at the site location), for example:

-   -   Gate    -   Conductor    -   User-defined type.

Monitor type—the monitoring method that should be applied to the device,for example:

-   -   Process inspection    -   Width measurement    -   Diameter measurement    -   Minimum distance measurement between devices    -   Maximum distance measurement between devices    -   Three-dimensional profile measurement    -   Matched width measurement.

Monitoring spec—the required measurement specification, such as theminimum and/or maximum permissible value, or the maximum deviationallowed among different measurements.

Measurement spec—other measurement parameters, including the requiredmeasurement direction and user comments.

Monitoring motivation—provided by the design or verification engineer toindicate the reason for monitoring this particular site, which mayinclude:

-   -   Critical timing point in a circuit.    -   Critical alignment point (such as self-aligned contacts between        different circuit layers).    -   Matched transistors (which must have substantially identical        characteristics for proper circuit performance).    -   Matched resistors (like matched transistors).    -   Marginal to design rule—a site at which small process deviations        may lead to design rule violation at this site.    -   Dense layout—which may cause artifacts in the photolithography        process.    -   Isolated feature (i.e., a feature needed for isolation between        conductors).    -   OPC issue—when OPC is used to print structures with        sub-wavelength resolution, results may be unpredictable and        should be tested.    -   High mask error enhancement factor (MEEF)—may cause        “amplification” of errors in printing the reticle on the die.    -   Process dishing and erosion—problems that may occur in the CMP        process because of different structure densities in different        regions. Sparse regions may be more intensively eroded during        the CMP process, and will therefore be lower than the rest of        the die.    -   Process step coverage.    -   User-defined (other motivations not listed above).        These fields in the site data are listed here by way of example,        and the site records may include other field types, as well.

The PDP data described above may be generated in multiple stages of thedesign process. Referring to the work flow shown above in Table I, forexample, the following design steps are particularly useful in providingPDP data:

Layout vs. schematic—Identify sites and regions requiring inspection.

Design rule check (DRC)—Identify sites and regions containing marginalstructures, i.e., structures that are close to the design rule limits.(Exemplary methods for DRC-based site selection are described below.)This step can also be used to determine overlay sites, such asself-aligning contacts and alignment targets, and sites for teststructures, in addition to or instead of designating these sites inearlier design stages.

Stream GDS2—Determine periodicity and density of different regions, aswell as the floor plan and size of the die and locations of alignmenttargets.

OPC—Extract sites for OPC verification.

PDP data may be generated at other steps, as well.

IMPLEMENTATION EXAMPLES

The following section lists a number of exemplary applications of theuse of PDP data generated by EDA workstations 22 and 23 (FIG. 1) indriving process monitor 36. These applications may advantageously usethe data structures defined above. Alternatively, the relevant PDP datamay be conveyed from the EDA tools to the appropriate process monitoringtools in other formats.

Selection of Inspection and Measurement Locations

The PDP data may be used in selecting the locations on die 34 that areto be examined in a given monitoring step. This feature of the presentinvention is useful, for example, in automatic process inspection usinge-beam and optical inspection tools, in critical dimension (CD)measurements and in other process monitoring applications. Typically,the locations for monitoring are those that are most susceptible toprocess faults, and are selected based on sites 56 that have beendesignated in the design and verification stages. As a result of thisguided site selection, the monitoring sensitivity in critical areas ofthe die is improved. By-products of this approach may include reducedset-up time in developing monitoring recipes in the fab and improvedutilization of existing monitoring tools. In addition, when themeasurement sites are pre-selected by the designer, it may be possibleto apply tighter control limits to the dimensions and to other circuitcharacteristics that are measured, thus increasing process yield.

Site-specific measurements can also be useful in providing feedback tothe designers on the manufacturability of their designs.

As noted above, design rule checking in the EDA stage may be used inchoosing sites to inspect in the fab. For example, a designer may runthe layout of a given IC layer through a design rule checker (DRC)program, which marks the locations in the layout that violate the designrules provided by the fab where the IC is to be produced. The designermay then widen or narrow dimensions of certain objects in theX-direction, Y-direction, or both, and may then apply the DRC again(with the same design rules). Any new locations that are found toviolate the design rules may be chosen as sites to list in the PDP. Thesteps of widening or narrowing dimensions and adding new sites may berepeated one or more times. Similarly, the threshold of the DRC forrecording a violation of the design rules may be decreased in order toadd sites to the PDP. If desired, the number of sites to be checked maybe set in advance, and the DRC threshold may then be adjustedautomatically in order to give the appropriate, preset number of sites.

An optical and process rule checking tool, such as the Calibre® ORC™tool made by Mentor Graphics (Wilsonville, Oreg.) may be used in asimilar fashion to identify sites for inspection. This tool simulatesthe production processes in the fab in order to verify that designstructures will be properly printed on the wafer. Like the DRC describedabove, the ORC tool may be used to place inspection sites at locationsat which violations are detected upon widening or narrowing dimensionsin the design. Alternatively or additionally, the designer may increaseor decrease other variable process parameters used in the ORC simulationin order to choose the inspection sites. For example, the designer maychange the focus exposure setting of the ORC simulation, and may thenplace inspection sites at locations at which violations occur at thedifferent settings. This latter method is useful in identifying sitesthat are sensitive to excursions of the process window. Otherlithography and etch parameters used in the simulation may likewise bevaried for this purpose.

FIG. 5 is a schematic detail view showing conductors 90 formed on die34, illustrating another application of site-specific monitoring, inaccordance with an embodiment of the present invention. CD measurementsknown in the art generally examine the widths of the conductors and thespacing between them. In the present example, however, it has beendetermined that the shapes and spacing of non-contacting ends 92 ofadjacent conductors are particularly susceptible to systematic processproblems, even when the critical dimensions along the lengths of theconductors are satisfactory. Therefore, the designer indicates one ormore sites 94 in the design of the appropriate reticle at which suchnon-contacting ends occur. In production of the wafer, the spacing andother characteristics of ends 92 may be measured against the monitoringspec for site 94, and the production processes associated with thereticle in question may be adjusted accordingly, if necessary.

Sites that are critical to chip functionality, reliability orperformance may be monitored in similar fashion. These are typicallysites at which a small process distortion is likeliest to cause a devicefailure. These sites may be identified in early stages of the designworkflow, such as the stages of circuit synthesis, simulation and timinganalysis. For example, sites that affect the clock rating of the chipand sites of other critical-path driver transistors (on clock lines andbuses) may be designated for monitoring. Other sorts of critical areasare described by Levasseur et al., in “Application of a Yield ModelMerging Critical Areas and Defectivity Data to Industrial Products,”Proceedings of the IEEE International Symposium on Defect and FaultTolerance in VLSI Systems (1997), pages 11-19, which is incorporatedherein by reference.

As another example, which was mentioned above, the designer may indicatethe sites of pairs of matched transistors for measurement of criticalcharacteristics, such as the oxide area. Transistor matching can beparticularly crucial in the yield of analog circuits, such asoperational amplifiers and push-pull amplifier stages. The criticalcharacteristics of the paired elements must be substantially identical,to within a prescribed tolerance. The importance of transistor matchingis described further by Pelgrom et al., in “Transistor Matching inAnalog CMOS Applications,” IEEE International Electron Devices Meeting(IEDM 98, 1998), pages 915-918, which is incorporated herein byreference.

Sites may further be selected based on the other “monitoringmotivations” listed in the preceding section, including sites of thefollowing types:

Non-redundant areas and devices, particularly in memory chips.

Extremely dense or extremely isolated features.

Critical alignment structures, such as self-aligned contacts.

Locations of high MEEF.

OPC locations.

Regions within the design of an IC that have relatively low aerial-imagecontrast are typically characterized by high MEEF, i.e., largeamplification of reticle errors. Inspection of high-MEEF sites cantherefore be useful in detecting reticle errors and preventing yieldloss that may result from such errors. A method that can be used in thiscontext for selecting measurement points with high MEEF values isdescribed, for example, by Adrichem et al., in “Mask ErrorEnhancement-Factor (MEEF) Metrology Using Automated Scripts in CATS,”Proceedings of the 22nd Annual BACUS Symposium on Photomask Technology(SPIE, Monterey, Calif., 2002), which is incorporated herein byreference.

As noted above, OPC is used in correcting for optical proximity effectsthat occur in printing structures of sub-wavelength dimensions onwafers. Simulations of sub-wavelength lithography that can be performedby EDA tools are not entirely accurate, and the results of OPC may varydepending on the shape of the structure and the background against whichit is printed. To address this problem, sites may be selected formonitoring, based on the OPC stage of the design process, in order toprovide representative samples of a range of different OPC conditions.For example, OPC sites with different structures may be chosen.Additionally or alternatively, it is possible to choose a group of OPCsites that have substantially the same structure, but are surrounded bydifferent background structures, or are surrounded by structures ofdifferent densities (dense vs. sparse structures).

Use of Periodic Regions in Setting Monitoring Parameters

As noted above, regions of mask 48 are classified by pitch 62 (FIG. 3),which indicates not only the spacing of periodic patterns in the region,but also the direction of the lines that make up such patterns. Forexample, a periodic structure, based on horizontal lines, is shown inFIG. 5.

When a sample, such as a wafer or mask, is illuminated by coherent lightin an optical inspection system, periodic patterns on the samplegenerate constructive interference lobes along well-defined directions.The position and extent of the interference lobes depend on the periodand direction of the pattern. The interference lobes may be blockedusing a suitable spatial (Fourier) filter, as is known in the art.Blocking out the interference lobes facilitates the detection of defectsand pattern irregularities on the sample.

As illustrated above in FIGS. 3 and 4, the PDP data provided by EDAtools makes it possible to divide each die into regions according topitch 62 and according to the direction of the pattern of the mask inthe region. Based on this information, the spatial filter for eachregion may be precomputed, rather than chosen by trial and error. Thisapplication of the PDP data reduces the set-up time involved in opticalinspection recipes and may improve the sensitivity of optical detectionby allowing the optimal Fourier filter to be designed and used in eachcase.

The direction and the density of the pattern in each region of the maskalso define the level of sensitivity that can be applied in inspectionof the corresponding region of the wafer. The sensitivity is expressed,for example, in terms of inspection parameters such as the beam energyand spot size that can be used to irradiate the wafer for inspection,detector sensitivity settings and detection algorithm thresholds. Whenthe same sensitivity is used over the entire wafer, as in systems knownin the art, the least sensitive setting (determined by the “noisiest”region of the wafer) must be applied to the entire wafer. By separatingthe wafer into regions according to pattern direction and density, it ispossible to increase the sensitivity in regions that do not contain anoisy pattern, and thus to detect more defects in these quiet regionswithout increasing the number of false alarms in the noisy regions.

The direction of the periodic pattern in a given region may be used tooptimize monitoring procedures in other ways, as well. For example, inon-the-fly optical classification of defects, the directionality of thepattern may be used to enhance the accuracy of automatic defectclassification. The classification is typically made according to theintensity of light received by detectors that are located around thewafer at different angles. The amount of light that reaches eachdetector depends not only on the defect itself, but also on the patterndirection in the region around the defect. The known pattern directionscan thus be used to achieve higher accuracy in the classificationalgorithm. Furthermore, a defect may often be classified as “killer” or“non-killer” depending on the region in which the defect was found.

The scan direction (horizontal or vertical) of the inspection headrelative to the wafer in each region can also be selected automaticallyaccording to the pattern direction. Generally, it is desirable to scanparallel to the lines of the pattern, so that the Fourier filter canremain fixed during the scan, as well as to facilitate die-to-dieregistration for comparison purposes in both optical and SEM-basedinspection.

Identification of Repeating Cells

PDP data regarding regions of exact period 60 may be used automaticallyor manually to identify the regions of the die that contain a matrix ofidentical cells. As noted above in reference to FIG. 3, these regionscan be inspected by cell-to-cell or cell-to-reference comparison,typically using a defect review SEM. The use of cell-to-cell comparison(as opposed to die-to-die comparison) is useful in increasing thethroughput of the SEM inspection system.

Die-to-Database Defect Detection

It is also possible to use the PDP data in order to compare images ofdies on the wafer to the mask design, instead of using die-to-diecomparison for defect detection, as in systems known in the art. Indie-to-database detection using SEM images, for example, each SEM imageis typically compared to the design by subtracting the edges in thedesign from the edges in the SEM image. This approach is useful inincreasing inspection throughput, as well as in detecting defects thatrecur on multiple dies, and thus would not be detected by die-to-diecomparison (for example, a particle on the mask that causes adeformation of the pattern on the wafer).

Automated Alignment

As shown in FIG. 2, the PDP data may include locations of alignmenttargets 50 for each mask 48. Monitoring tools may also receive the sizesand locations of dies 34 on wafer 32 either from design database 24(FIG. 1), or from another source in the fab. Using this information, theexact locations of all relevant alignment targets on the wafer may bedetermined automatically for monitoring purposes, obviating the need formanual input of these parameters by the process engineer.

Setting Defect Detection Parameters

As noted above in reference to FIG. 3, the criticality classification ofa region of the die is typically indicative of the maximum tolerabledefect size in the region. Defects on the reticle or on the wafer thatare larger than this maximum tolerable size are likely to connectseparate circuit features adjacent to the defect. The criticalityparameter in each section may thus be used to set the sensitivitythreshold for defect detection in an optical inspection system or defectreview SEM. In regions with high criticality, the threshold may be setlow, in order to permit detection of very small particles. In regionswith low criticality, it may be possible to forgo inspection entirely.Controlling the detection sensitivity in this manner permits throughputto be maximized—by avoiding unnecessary inspection of non-criticalareas—while focusing detection resources where and as they are needed.

Additionally or alternatively, the criticality classification may beused in assigning a “killer index” to each defect, indicating theexpected functional effect of the defect on the chip. The functionaleffect will generally depend not only on criticality, but also on otherregion classification factors, such as function and redundancy. Thesefactors may also be used in selecting certain defects for furtherreview, either by the inspection tool itself or by a process engineer,or both.

Region segmentation based on PDP data may also be used in automaticdefect classification (ADC), so as to eliminate the manual segmentationprocedure that is generally required in order to tune the ADC algorithmin defect review systems known in the art. For example, the regioninformation may be used to automatically set variable gray-levelthresholds that are used to detect and classify defects in differentregions of the wafer.

Selecting Test Structures and Procedures

FIG. 6 is a schematic top view of a structure 100 on a semiconductorwafer. The structure is used in electrical testing of the wafer, inaccordance with an embodiment of the present invention. Structure 100may be identified as one of special structures 52 in the PDP data (FIG.2), and may be provided specifically for the purpose of wafer testing,typically in one of non-die areas 46. Alternatively, the structure mayhave a functional purpose inside a die, for example, as a word line ordata bus, which happens to be particularly amenable to a certain kind oftesting.

In the example shown in FIG. 6, structure 100 comprises parallelconducting lines 102 on a substrate 104. Based on the PDP data, charginglocations 106 are set at one end of lines 102, and measurement locations108 are set at the opposite end. Charging locations 106 are charged, bya focused electron beam, for instance, and the charge on lines 102 ismeasured at measurement locations 106. The charge may be measured, forexample, using a voltage contrast technique in a SEM, wherein thegray-scale level of structures in the SEM image indicates whether or notthey are electrically charged. Alternatively, other charging andmeasurement techniques may be used.

Typically, not all of lines 102 are charged. Rather, in the presentexample, every other line is charged. The charge on the non-chargedlines may also be measured at one or more alternative measurementlocations 112. Normally, no charge should be measured at thesealternative locations. In FIG. 6, however, there is a short circuit 110between one of the charged lines and the adjacent uncharged line. Acharge will therefore be detected at location 112, indicating that thereis a defect in structure 100. On the other hand, a discontinuity 114 inone of lines 102 will result in an open circuit, so that a correspondingmeasurement location 116 will not exhibit the expected charge.

The method illustrated in FIG. 6 thus allows large areas of a die to betested quickly without actually scanning the entire area. Thiselectrical testing technique can detect the existence of any criticalfault anywhere along the length of structure 100. Although FIG. 6 shows,by way of illustration, a particular, simple structure, the methodexemplified by the figure may be used for testing all sorts of elongatedconducting structures on the wafer.

Another type of electrical testing that can be facilitated by themethods of the present invention is specimen current measurement usingan electron beam, for evaluating certain structures on the wafer, suchas contact holes. The specimen current in an electron beam system isdefined as the absorbed current that flows from the primary electronbeam to ground via the wafer. It has been found that the specimencurrent is a good indicator of the etch quality of contact holes, aswell as of other types of contact openings, such as vias and trenches.Exemplary methods of contact hole testing based on specimen currentmeasurement are described, for example, in U.S. patent application Ser.No. 10/209,087, filed Jul. 30, 2002, and in U.S. patent application Ser.No. 10/434,977, filed May 9, 2003, which are assigned to the assignee ofthe present patent application and are incorporated herein by reference.In an embodiment of the present invention, a recipe for specimencurrent-based testing of a wafer is created automatically, by selectingmeasurement locations and other measurement parameters based on theknown, three-dimensional wafer structure.

The data exchange between the EDA environment and the fab permitselectrical tests to be built into the reticle design and then executedautomatically in the fab based on the PDP data. For example, certainlines on a die may be connected to pads, which may then be accessed byan electrical tester. The pads may be placed in non-die areas, so thatwhen the wafer is cut, the connections are isolated.

Yield Prediction Based on Design Information

Yield enhancement is a key objective of process monitoring in the fab.For purposes of yield enhancement, it is useful to be able to estimatethe effect of certain process deviations on the actual IC yield of theoverall process. Statistical methods have been developed for relatingdesign and process parameters to the yield. For example, theabove-mentioned article by Levasseur et al. a method for yield modelingbased on combining the computation of critical areas (a measure of thesusceptibility of a device to particle defects) with measurement of theactual defects detected by process inspection.

In some embodiments of the present invention, PDP data are used togetherwith inspection results as statistical inputs in predicting andoptimizing IC yield. As noted above, the PDP data may be used toidentify critical sites on the die, at which process deviations mayresult in device failure. By appropriate selection of these sites, themeasurements made by monitoring tools at these sites (for example, meandimensions and standard deviation of critical structures at a number oflocations) may be used in predicting the process yield. Suchmeasurements may be made on a single die or, preferably, over multipledies on one or more wafers. Additionally or alternatively, teststructures for process characterization and yield prediction may begenerated automatically from the PDP data, along with test parametersfor evaluating production of the test structures.

CONCLUSION

Although the embodiments described above relate to certain specificimplementation examples, the principles of the present invention maysimilarly be applied to automated generation of other sorts of processmonitoring tests and evaluation of their results. Furthermore, PDP datamay be used not only in monitoring, but also in automated selection ofprocess tool recipes.

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsubcombinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art.

1. Apparatus for producing an integrated circuit (IC) on a semiconductorwafer, comprising: an electronic design automation (EDA) tool, which isadapted to generate a design of a layer of the IC and a product designprofile (PDP), which comprises an indication of a site in the layer thatis susceptible to a process fault; a production tool, which is adaptedto fabricate the layer of the IC on the wafer responsively to thedesign; and a process monitoring tool, which is adapted to perform ameasurement at the site in the layer responsively to the PDP.
 2. Theapparatus according to claim 1, wherein the process monitoring tool isadapted to measure a dimension associated with one or more features ofthe IC at the site.
 3. The apparatus according to claim 1, wherein theindication is included in the PDP responsively to a determination thatthe site is critical to a performance rating of the IC.
 4. The apparatusaccording to claim 1, wherein the indication is included in the PDPresponsively to a determination that the site is marginal with respectto a design rule of the IC.
 5. The apparatus according to claim 1,wherein the indication is included in the PDP responsively to adetermination that the site is marginal with respect to a variation in aparameter of a process used in fabricating the IC.
 6. The apparatusaccording to claim 1, wherein the EDA tool is adapted to determine amask error enhancement factor (MEEF) at the site, and to select the siteresponsively to the MEEF.
 7. The apparatus according to claim 1, whereinthe EDA tool is adapted to determine an optical proximity correction(OPC) to be applied at the site, and to select the site responsively tothe OPC.
 8. The apparatus according to claim 1, wherein the EDA tool isadapted to select the site responsively to a density of structures inthe IC at the site.
 9. The apparatus according to claim 1, wherein thesite comprises a location of a pair of matched circuit elements, andwherein the process monitoring tool is adapted to verify that a criticalcharacteristic of both the circuit elements in the pair is substantiallyidentical.
 10. The apparatus according to claim 1, and comprising acomputer processor, which is adapted to predict a yield of thefabrication of the IC responsively to the PDP and to the measurement.11. Apparatus for producing an integrated circuit (IC) on asemiconductor wafer, comprising: an electronic design automation (EDA)tool, which is adapted to generate a design of a layer of the IC and aproduct design profile (PDP), which comprises an identification of aregion in a layer of the IC that is characterized by a periodic pattern;a production tool, which is adapted to fabricate the layer of the IC onthe wafer responsively to the design; and a process monitoring tool,which is adapted to perform a measurement in the region in the layerresponsively to the periodic pattern.
 12. The apparatus according toclaim 11, wherein the PDP comprises an indication of a pitch and adirection of the periodic pattern, and wherein the process monitoringtool is adapted to perform optical inspection of the region using aspatial filter, wherein the spatial filter is selected responsively tothe pitch and the direction indicated by the PDP.
 13. The apparatusaccording to claim 11, wherein the PDP comprises an indication of adirection of the periodic pattern, and wherein the process monitoringtool is adapted to inspect the region while scanning over the region inthe selected scan direction, wherein the scan direction is selectedresponsively to the direction of the periodic pattern indicated by thePDP.
 14. The apparatus according to claim 11, wherein the PDP comprisesan indication of an exact period of a repetitive feature in the periodicpattern, and wherein the process monitoring tool is adapted to capturemultiple images of the feature at selected locations on the wafer and tocompare each of the images to another of the images or to a referenceimage, wherein the selected locations are mutually spaced by the exactperiod.
 15. The apparatus according to claim 11, wherein the processmonitoring tool has a variable sensitivity setting, wherein differentsensitivity settings are applied by the process monitoring tool fordetecting defects in different regions of the layer, and wherein thesensitivity setting to be applied in the region is determinedresponsively to the periodic pattern.
 16. Apparatus for producing anintegrated circuit (IC) on a semiconductor wafer, comprising: anelectronic design automation (EDA) tool, which is adapted to generate adesign of a layer of the IC and a product design profile (PDP), whichcomprises an identification of a plurality of regions in a layer of theIC and a respective criticality parameter for each of the regions,indicative of a maximum tolerable defect size in each of the regions; aproduction tool, which is adapted to fabricate the layer of the IC onthe wafer responsively to the design; and a process monitoring tool,which is adapted to perform a measurement in one or more of the regionsresponsively to the respective criticality parameter.
 17. The apparatusaccording to claim 16, wherein a defect detection threshold of theprocess monitoring tool is set in each of the one or more of the regionsresponsively to the respective criticality parameter.
 18. The apparatusaccording to claim 16, wherein the one or more of the regions to inspectare selected responsively to the respective criticality parameter. 19.The apparatus according to claim 16, wherein the process monitoring toolis adapted to detect a defect in one of the regions, and to classify thedefect responsively to the criticality parameter.
 20. Apparatus forproducing an integrated circuit (IC) on a semiconductor wafer,comprising: an electronic design automation (EDA) tool, which is adaptedto generate a design of a layer of the IC, the layout comprising astructure that is amenable to testing, and which is further adapted togenerate and a product design profile (PDP), which comprises informationregarding the structure; a production tool, which is adapted tofabricate the layer of the IC on the wafer responsively to the design;and a process monitoring tool, which is adapted to perform a measurementon the structure, responsively the information in the PDP.
 21. Theapparatus according to claim 20, wherein the structure comprises adedicated test structure.
 22. The apparatus according to claim 21,wherein the dedicated test structure is located in a non-die area of thewafer.
 23. The apparatus according to claim 20, wherein the structurecomprises multiple elongate parallel conductors, and wherein the processmonitoring tool is adapted to test an electrical continuity of theconductors.
 24. The apparatus according to claim 23, wherein the processmonitoring tool is adapted to test the electrical continuity by applyingan electrical charge at a first end of at least some of the conductors,and measuring the electrical charge at a second end of the conductors.25. The apparatus according to claim 20, wherein the structure comprisesone or more contact openings in the layer, and wherein the processmonitoring tool is adapted to direct an electron beam to irradiate theone or more contact openings, and to measure a specimen currentresponsively to the electron beam.
 26. A computer software product foruse in producing an integrated circuit (IC) on a semiconductor wafer,the product comprising a computer-readable medium in which programinstructions are stored, the instructions comprising at least one of anelectronic design automation (EDA) program component and a processmonitoring program component, wherein the EDA program component, whenread by a computerized EDA tool, causes the EDA tool to generate adesign of a layer of the IC and a product design profile (PDP), whichcomprises an indication of a site in the layer that is susceptible to aprocess fault, and wherein the process monitoring program component,when read by a computerized process monitoring tool after fabrication ofthe layer of the IC on the wafer responsively to the design, causes theprocess monitoring tool to perform a measurement at the site in thelayer responsively to the PDP.
 27. The product according to claim 26,wherein the process monitoring program component causes the processmonitoring tool to measure a dimension associated with one or morefeatures of the IC at the site.
 28. The product according to claim 26,wherein the EDA program component causes the EDA tool to make adetermination that the site is critical to a performance rating of theIC, and to include the site in the PDP responsively to thedetermination.
 29. The product according to claim 26, wherein the EDAprogram component causes the EDA tool to make a determination that thesite is marginal with respect to a design rule of the IC, and to includethe site in the PDP responsively to the determination.
 30. The productaccording to claim 26, wherein the EDA program component causes the EDAtool to make a determination that the site is marginal with respect to avariation in a parameter of a process used in fabricating the IC, and toinclude the site in the PDP responsively to the determination.
 31. Theproduct according to claim 26, wherein the EDA program component causesthe EDA tool to determine a mask error enhancement factor (MEEF) at thesite, and to select the site responsively to the MEEF.
 32. The productaccording to claim 26, wherein the EDA program causes the EDA tool todetermine an optical proximity correction (OPC) to be applied at thesite, and to select the site responsively to the OPC.
 33. The productaccording to claim 26, wherein the EDA program causes the EDA tool toselect the site responsively to a density of structures in the IC at thesite.
 34. The product according to claim 26, wherein the site comprisesa location of a pair of matched circuit elements, and wherein theprocess monitoring program component causes the process monitoring toolto verify that a critical characteristic of both the circuit elements inthe pair is substantially identical.
 35. The product according to claim26, wherein the process monitoring program component further causes theprocess monitoring tool to predict a yield of the fabrication of the ICresponsively to the PDP and to the measurement.
 36. A computer softwareproduct for use in producing an integrated circuit (IC) on asemiconductor wafer, the product comprising a computer-readable mediumin which program instructions are stored, the instructions comprising atleast one of an electronic design automation (EDA) program component anda process monitoring program component, wherein the EDA programcomponent, when read by a computerized EDA tool, causes the EDA tool togenerate a design of a layer of the IC and a product design profile(PDP), which comprises an identification of a region in a layer of theIC that is characterized by a periodic pattern, and wherein the processmonitoring program component, when read by a computerized processmonitoring tool after fabrication of the layer of the IC on the waferresponsively to the design, causes the process monitoring tool toperform a measurement in the region in the layer responsively to theperiodic pattern.
 37. The product according to claim 36, wherein the EDAprogram component causes the EDA tool to include in the PDP anindication of a pitch and a direction of the periodic pattern, so thatthe process monitoring tool performs optical inspection of the regionusing a spatial filter that is selected responsively to the pitch andthe direction indicated by the PDP.
 38. The product according to claim36, wherein the EDA program component causes the EDA tool to include inthe PDP an indication of a direction of the periodic pattern, andwherein the process monitoring program component causes the processmonitoring tool to inspect the region while scanning over the region inthe selected scan direction, wherein the scan direction is selectedresponsively to the direction of the periodic pattern indicated by thePDP.
 39. The product according to claim 36, wherein the EDA programcomponent causes the EDA tool to include in the PDP an indication of anexact period of a repetitive feature in the periodic pattern, andwherein the process monitoring program component causes the processmonitoring tool to capture multiple images of the feature at selectedlocations on the wafer and to compare each of the images to another ofthe images or to a reference image, wherein the selected locations aremutually spaced by the exact period.
 40. The product according to claim36, wherein the process monitoring tool has a variable sensitivitysetting, and wherein the process monitoring program component causes theprocess monitoring tool to apply different sensitivity settings indetecting defects in different regions of the layer, wherein thesensitivity setting to be applied in the region is determinedresponsively to the periodic pattern.
 41. A computer software productfor use in producing an integrated circuit (IC) on a semiconductorwafer, the product comprising a computer-readable medium in whichprogram instructions are stored, the instructions comprising at leastone of an electronic design automation (EDA) program component and aprocess monitoring program component, wherein the EDA program component,when read by a computerized EDA tool, causes the EDA tool to generate adesign of a layer of the IC and a product design profile (PDP), whichcomprises an identification of a plurality of regions in a layer of theIC and a respective criticality parameter for each of the regions,indicative of a maximum tolerable defect size in each of the regions,and wherein the process monitoring program component, when read by acomputerized process monitoring tool after fabrication of the layer ofthe IC on the wafer responsively to the design, causes the processmonitoring tool to perform a measurement in one or more of the regionsresponsively to the respective criticality parameter.
 42. The productaccording to claim 41, wherein the process monitoring program componentcauses a defect detection threshold of the process monitoring tool to beset in each of the one or more of the regions responsively to therespective criticality parameter.
 43. The product according to claim 41,wherein the one or more of the regions to inspect are selectedresponsively to the respective criticality parameter.
 44. The productaccording to claim 41, wherein the process monitoring program componentcauses the process monitoring tool to detect a defect in one of theregions, and to classify the defect responsively to the criticalityparameter.
 45. A computer software product for use in producing anintegrated circuit (IC) on a semiconductor wafer, the product comprisinga computer-readable medium in which program instructions are stored, theinstructions comprising at least one of an electronic design automation(EDA) program component and a process monitoring program component,wherein the EDA program component, when read by a computerized EDA tool,causes the EDA tool to generate a design of a layer of the IC, thelayout comprising a structure that is amenable to testing, and a productdesign profile (PDP), which comprises information regarding thestructure, and wherein the process monitoring program component, whenread by a computerized process monitoring tool after fabrication of thelayer of the IC on the wafer responsively to the design, causes theprocess monitoring tool to perform a measurement on the structure,responsively the information in the PDP.
 46. The product according toclaim 45, wherein the structure comprises a dedicated test structure.47. The product according to claim 46, wherein the dedicated teststructure is located in a non-die area of the wafer.
 48. The productaccording to claim 45, wherein the structure comprises multiple elongateparallel conductors, and wherein the process monitoring programcomponent causes the process monitoring tool to test an electricalcontinuity of the conductors.
 49. The product according to claim 48,wherein the process monitoring tool causes the process monitoring toolto test the electrical continuity by applying an electrical charge at afirst end of at least some of the conductors, and measuring theelectrical charge at a second end of the conductors.
 50. The productaccording to claim 45, wherein the structure comprises one or morecontact openings in the layer, and wherein the process monitoringprogram component causes the process monitoring tool to direct anelectron beam to irradiate the one or more contact openings, and tomeasure a specimen current responsively to the electron beam. 51.Apparatus for monitoring production of an integrated circuit (IC) on asemiconductor wafer, following fabrication of a layer of the IC on thewafer responsively to a design and a product design profile (PDP)generated by an electronic design automation (EDA) tool, the PDPincluding an indication of a site in the layer that is susceptible to aprocess fault, the apparatus comprising: a process monitoring tool,which is adapted to perform a measurement at the site in the layerresponsively to the PDP.
 52. The apparatus according to claim 51,wherein the process monitoring tool is adapted to measure a dimensionassociated with one or more features of the IC at the site.
 53. Theapparatus according to claim 51, wherein the indication in the PDPindicates that the site is critical to a performance rating of the IC.54. The apparatus according to claim 51, wherein the indication in thePDP indicates that the site is marginal with respect to a design rule ofthe IC.
 55. The apparatus according to claim 51, wherein the indicationin the PDP indicates that the site is marginal with respect to avariation in a parameter of a process used in fabricating the IC. 56.The apparatus according to claim 51, wherein the site comprises alocation of a pair of matched circuit elements, and wherein the processmonitoring tool is adapted to verify that a critical characteristic ofboth the circuit elements in the pair is substantially identical. 57.The apparatus according to claim 51, and comprising a computerprocessor, which is adapted to predict a yield of the fabrication of theIC responsively to the PDP and to the measurement.